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Scala 3 Migration Guide

Other Changed Features

Language

Some other features are simplified or restricted to make the language easier, safer or more consistent.

Incompatibility Scala 3 Migration Rewrite
Inheritance shadowing
Non-private constructor in private class Migration Warning
Abstract override  
Case class companion  
Explicit call to unapply  
Invisible bean property  
=>T as type argument  
Wildcard type argument  

Inheritance Shadowing

An inherited member, from a parent trait or class, can shadow an identifier defined in an outer scope. That pattern is called inheritance shadowing.

object B {
  val x = 1
  class C extends A {
    println(x)
  }
}

For instance, in this preceding piece of code, the x term in C can refer to the x member defined in the outer class B or it can refer to a x member of the parent class A. You cannot know until you go to the definition of A.

This is known for being error prone.

That’s why, in Scala 3, the compiler requires disambiguation if the parent class A does actually have a member x.

It prevents the following piece of code from compiling.

class A {
  val x = 2
}

object B {
  val x = 1
  class C extends A {
    println(x)
  }
}
-- [E049] Reference Error: src/main/scala/inheritance-shadowing.scala:9:14 
9 |      println(x)
  |              ^
  |              Reference to x is ambiguous,
  |              it is both defined in object B
  |              and inherited subsequently in class C

The Scala 3 migration compilation can automatically disambiguate the code by replacing println(x) with println(this.x).

Non-private Constructor In Private Class

The Scala 3 compiler requires the constructor of private classes to be private.

For instance, in the example:

package foo

private class Bar private[foo] () {}

The error message is:

-- Error: /home/piquerez/scalacenter/scala-3-migration-guide/incompat/access-modifier/src/main/scala-2.13/access-modifier.scala:4:19 
4 |  private class Bar private[foo] ()
  |                   ^
  |      non-private constructor Bar in class Bar refers to private class Bar
  |      in its type signature (): foo.Foo.Bar

The Scala 3 migration compilation warns about this but no automatic rewrite is provided.

The solution is to make the constructor private, since the class is private.

Abstract Override

In Scala 3, overriding a concrete def with an abstract def causes subclasses to consider the def abstract, whereas in Scala 2 it was considered as concrete.

In the following piece of code, the bar method in C is considered concrete by the Scala 2.13 compiler but abstract by the Scala 3 compiler, causing the following error.

trait A {
  def bar(x: Int): Int = x + 3
}

trait B extends A {
  def bar(x: Int): Int
}

class C extends B // Error: class C needs to be abstract, since def bar(x: Int): Int is not defined

This behavior was decided in Dotty issue #4770.

An easy fix is simply to remove the abstract def, since in practice it had no effect in Scala 2.

Case Class Companion

The companion object of a case class does not extend any of the Function{0-23} traits anymore. In particular, it does not inherit their methods: tupled, curried, andThen, compose

For instance, this is not permitted anymore:

case class Foo(x: Int, b: Boolean)

Foo.curried(1)(true)
Foo.tupled((2, false))

A cross-compiling solution is to explicitly eta-expand the method Foo.apply.

-Foo.curried(1)(true)
+(Foo.apply _).curried(1)(true)

-Foo.tupled((2, false))
+(Foo.apply _).tupled((2, false))

Or, for performance reasons, you can introduce an intermediate function value.

val fooCtr: (Int, Boolean) => Foo = (x, b) => Foo(x, b)

fooCtr.curried(1)(true)
fooCtr.tupled((2, false))

Explicit Call to unapply

In Scala, case classes have an auto-generated extractor method, called unapply in their companion object. Its signature has changed between Scala 2.13 and Scala 3.

The new signature is option-less (see the new Pattern Matching reference), which causes an incompatibility when unapply is called explicitly.

Note that this problem does not affect user-defined extractors, whose signature stays the same across Scala versions.

Given the following case class definition:

case class Location(lat: Double, long: Double)

The Scala 2.13 compiler generates the following unapply method:

object Location {
  def unapply(location: Location): Option[(Double, Double)] = Some((location.lat, location.long))
}

Whereas the Scala 3 compiler generates:

object Location {
  def unapply(location: Location): Location = location
}

Consequently the following code does not compile anymore.

def tuple(location: Location): (Int, Int) = {
  Location.unapply(location).get // [E008] Not Found Error: value get is not a member of Location
}

A possible solution is to use pattern binding:

def tuple(location: Location): (Int, Int) = {
-  Location.unapply(location).get
+  val Location(lat, lon) = location
+  (lat, lon)
}

Invisible Bean Property

The getter and setter methods generated by the BeanProperty annotation are now invisible in Scala 3 because their primary use case is the interoperability with Java frameworks.

For instance, in the below example:

class Pojo() {
  @BeanProperty var fooBar: String = ""
}

val pojo = new Pojo()

pojo.setFooBar("hello") // [E008] Not Found Error: value setFooBar is not a member of Pojo

println(pojo.getFooBar()) // [E008] Not Found Error: value getFooBar is not a member of Pojo

The solution is to call the more idiomatic pojo.fooBar getter and setter.

val pojo = new Pojo()

-pojo.setFooBar("hello")
+pojo.fooBar = "hello"

-println(pojo.getFooBar())
+println(pojo.fooBar)

=> T as Type Argument

A type of the form => T cannot be used as an argument to a type parameter anymore.

This decision is explained in this comment of the Scala 3 source code.

For instance, it is not allowed to pass a function of type Int => (=> Int) => Int to the uncurried method since it would assign => Int to the type parameter T2.

-- [E134] Type Mismatch Error: src/main/scala/by-name-param-type-infer.scala:3:41
3 |  val g: (Int, => Int) => Int = Function.uncurried(f)
  |                                ^^^^^^^^^^^^^^^^^^
  |None of the overloaded alternatives of method uncurried in object Function with types
  | [T1, T2, T3, T4, T5, R]
  |  (f: T1 => T2 => T3 => T4 => T5 => R): (T1, T2, T3, T4, T5) => R
  | [T1, T2, T3, T4, R](f: T1 => T2 => T3 => T4 => R): (T1, T2, T3, T4) => R
  | [T1, T2, T3, R](f: T1 => T2 => T3 => R): (T1, T2, T3) => R
  | [T1, T2, R](f: T1 => T2 => R): (T1, T2) => R
  |match arguments ((Test.f : Int => (=> Int) => Int))

The solution depends on the situation. In the given example, you can either:

  • define your own uncurried method with the appropriate signature
  • inline the implementation of uncurried locally

Wildcard Type Argument

Scala 3 cannot reduce the application of a higher-kinded abstract type member to the wildcard argument.

For instance, the following example does not compile.

trait Example {
  type Foo[A]

  def f(foo: Foo[_]): Unit // [E043] Type Error: unreducible application of higher-kinded type Example.this.Foo to wildcard arguments 
}

We can fix this by using a type parameter:

-def f(foo: Foo[_]): Unit
+def f[A](foo: Foo[A]): Unit

But this simple solution does not work when Foo is itself used as a type argument.

def g(foos: Seq[Foo[_]]): Unit

In such case, we can use a wrapper class around Foo:

+class FooWrapper[A](foo: Foo[A])

-def g(foos: Seq[Foo[_]]): Unit
+def g(foos: Seq[FooWrapper[_]]): Unit

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